1. Field of the Invention
This invention relates in general to rectangular waveguide with elbows (E-elbow) which are bent across the broad side of the waveguide with the outer corners symmetrically flattened by conductive flattening or smoothing planes.
2. Description of the Prior Art
Such elbows are described, for example, in "Taschenbuch der Hochfrequenztechnik", by H. Meinke and F. W. Fundlach, Springer Verlag, 2nd Edition, 1962, pages 401 and 402. Such elbows are utilized in various microwave circuits with rectangular waveguides. By using angled waveguides a more compact structure is achieved as compared to a comparable low-refraction circular arc elbow, particularly for use as waveguide shunts are filters of different types, such as for example fixed frequency shunts or filters, polarization shunts or filters or wave mode shunts or filters. Using waveguides with a rectangular cross-section having a side ratio of a:b equal to 2:1 are most often utilized. Such waveguides can be used in the relative frequency range with a maximum bandwidth of f.sub.o and f.sub.u =2:1 for the TE10 wave. Also, as discussed in the above reference publication "Taschenbuch der Hochfrequenztechnik" the reflection of an E-elbow can be reduced if, as shown in FIG. 1a, the exterior corner of the elbow is symmetrically flattened or smoothed with a conducting plane. FIG. 1b illustrates the standing wave ratio s at frequencies f for E-elbows as shown in FIG. 1a with corner flattening planes of varying degrees. The optimum cathetus measurement x.sub.o is shown in the lowest curve wherein the ratio of x.sub.o /a=0.395 wherein a is the width of the long side of the waveguide has been derived and described in the above referenced publication. With such ratio, the reflection of an E-elbow will remain under r=5% in the frequency range of a waveguide which will usually be 1.25 fcTE10 through 1.9 fcTE10. Only in partial frequency bands of this frequency range can smaller reflections be achieved and for this purpose, the cathetus dimension can be changed somewhat with respect to x.sub.o depending upon the position of the partial frequency band within the full frequency band of the waveguide.
Utilizing a side ratio of the rectangular waveguide a:b=2:1, FIG. 1b illustrates in detail how the respective SWRs of E-elbows change in a waveguide over a frequency range for an E-elbow with a bend angle of 90.degree. and a few selected ratios x/a of the corner flattening or smoothing plane. Without corner flattening wherein the ratio x/a=0, an E-elbow represents an inducive disturbance with respect to a cross-section plane lying in the median line of the bend which inductive disruption increases greatly from the lower toward the top of the frequency range of a rectangular waveguide as shown. With increasing corner flattening or smoothing, in other words, increasing the quotient x/a, the inductive disruption becomes less and less. When the corner flattening or smoothing reaches a point where the ratio x/a=0.395, equal disruptions of r=5% will remain at the lower and upper frequency limits of the waveguide frequency range. These disruptions will have opposite phase angles and therefore such reflections will not fall below this value using a corner flattening method of compensation. Such reflections still represent a significant disruption in many utilizations which are standard today can be attributed to the fact that the compensation measure of corner flattening or smoothing alone is not precisely complementary to the disruption which is to be compensated over the entire frequency range of a rectangular waveguide.
For further reduction of the reflection factor over a relatively broad frequency band, it has already been proposed to provide a conductive cylindrical cross-bar in the area of the geometrical median of the bend, with the cross-bar being aligned parallel to the broad sides of the waveguide and extending between the narrow sides of the waveguide which lie opposite each other and to provide the flattening or smoothing plane with a conductive means as for example, a metal cylinder which projects into the interior space of the waveguide in the area of its diagonal point of intersection. An E-elbow compensated in these manners have very low reflection over an entire frequency band of the waveguide, but, however, the cost of manufacturing such devices is expensive because of the three different compensation features.